1. Field of the Invention
The invention relates to an abnormality diagnostic device and an abnormality diagnostic method for an intake air temperature sensor for an internal combustion engine which diagnose the presence/absence of an abnormality regarding an intake air temperature sensor that detects the temperature of the intake air supplied to an intake pipe of an internal combustion engine that is mounted in a vehicle.
2. Description of the Related Art
In internal combustion engines for motor vehicles, the air-fuel ratio is controlled in order to make proper the output characteristic, the fuel economy characteristic, the emission characteristic, etc., under various conditions. In the air-fuel ratio control, the amount of filet injection for obtaining a proper air-fuel ratio is calculated on the basis of the rotation speed of the internal combustion engine and the amount of intake air. Although the intake air amount is always detected via an intake air amount sensor, the detected intake air amount does not always equal the actual volume thereof since intake air changes in density in accordance with the temperature. Therefore, the intake pipe into which intake air is introduced is provided with an intake air temperature sensor as well, and the value of the intake air amount detected via the intake air amount sensor is corrected on the basis of the temperature of the intake air detected by the intake air temperature sensor, so as to conform to the actual volume of the intake air. Therefore, in order to obtain proper air-fuel ratio, the temperature of the intake air needs to be accurately detected by the intake air temperature sensor.
The intake air temperature sensor, however, can possibly deteriorate in detection characteristic due to time-dependent changes. Furthermore, the intake air temperature sensor may have a fault that is caused by a break, a shortcircuit or the like in the electrical system of the sensor. At any rate, such an abnormality of the intake air temperature sensor can become a cause of the internal combustion engine failing to obtain a proper air-fuel ratio, and therefore the presence/absence of such an abnormality needs to be diagnosed with high reliability. Therefore, early diagnosis of the presence/absence of an abnormality of the intake air temperature sensor is pursued by introducing a diagnostic device as disclosed in, for example, Japanese Patent Application Publication No. JP-A-10-61479. The diagnostic device described in Japanese Patent Application Publication No. JP-A-10-61479 diagnoses the presence/absence of an abnormality of the intake air temperature sensor on the assumption that when the vehicle is at a stop with the engine being in a fully warmed-up state, the intake air temperature detected via the intake air temperature sensor should be a correspondingly high temperature.
By executing the diagnosis of the intake air temperature sensor in this manner, the reliability of the detection of the intake air temperature can be kept high at the time of the air-fuel ratio control as well. Even if the presence of an abnormality is diagnosed, a safety run of the vehicle to a safety place or the like can be carried out on the basis of a predetermined appropriate fail-safe process. However, from the standpoint of early diagnosis with the air-fuel ratio control and the like taken into consideration, it is desirable that the diagnosis be performed at the time point when the internal combustion engine is started up even though the engine is under a cold condition or the like. In this respect, if the fully warmed-up state of the internal combustion engine is awaited before the diagnosis is executed as in the diagnostic device described in Japanese Patent Application Publication No. JP-A-10-61479, it is difficult to determine whether the engine control, in particular, the air-fuel ratio control or the like, has been properly performed during the wait.
Aside from the desirable construction in which the diagnosis is performed at the time point when the internal combustion engine is started up, the temperature in the engine compartment of the vehicle can become considerably high during the stop of the engine following the previous trip, due to the effect of sun shine or the like for example, in the case where the vehicle is parked in the sun for a long time. In that case, the temperature of the internal combustion engine, in particular, of the intake pipe where the intake air temperature sensor is mounted, can rise considerably high. That is, if the diagnosis of the intake air temperature sensor is executed under such a situation, proper diagnosis of the presence/absence of an abnormality of the intake air temperature cannot necessarily be made through the temperature information detected via the sensor.